BGE: unwanted patterns in ground texture from repeated, seamless image
It is the nature of a grid to repeat the same pattern in a recognizable way. What you need is something that breaks the pattern.

Adding variations to the size does not really help as our brain can easily handle different scales. 

Changing rotation can help more as long as the rotation is good enough. But this usually breaks the "seamless".

You can add variations by using more than one tile type. When you have 5+ with random order the repetition is much less noticeable.

Another option is to combine different tiles. This means you mix different tiles to confuse our brain. The repetition of the different "layers" should have a different scale. This way you can get easily a large field with just a few texture tiles that does not look repetitive (like dirt on the ground).

These are just a few ideas.